[Dovecot] Live Mirror dovecot / OS X 10.6.4 (Timo Sirainen)

[Timo Sirainen]

On Wed, 2010-08-25 at 05:43 -0400, Eric Boltz (MMI / UPC) wrote:
> > 1. If users can do any changes on the secondary server (flag changes,
> > saving new mails, etc), you're going to have problems syncing the
> > changes back (without losing any changes or causing other problems).
> >
> Change in strategy.
> Primary rsyncs regularly to the secondary which keeps all mail services 
> off. When primary fails email services are started manually until 
> primary back online. 

Okay..

> Then I (or another ITer) manually stop the backup, 
> rsycn and restart services on the primary. 

But if secondary server allowed changes, this is the difficult step to
do correctly. For example lets say in first server:

1. It rsyncs user foo
2. User foo changes message A flag to \Seen
3. User foo deletes message B
4. User foo saves a new message C
5. The server crashes
6. Secondary server goes up, but 2-4 changes weren't resynced yet
7. User foo logs in and changes message D's flag to \Seen
8. Primary server goes back up
9. You rsync .. how? If you make primary server look exactly like
secondary server, you message C gets lost. If you copy new files, now
message A and message D gets duplicated, because their filename changed
due to the flag change. Also message B becomes undeleted.

Of course assuming this happens rarely enough, those aren't a huge
problem, but the server move isn't always transparent to users and
you'll get errors in Dovecot's logs.